A humidifier is a device that increases humidity (moisture) in a single room or an entire building. Point-of-use humidifiers are commonly used to humidify a single room, while whole-house or furnace humidifiers, which connect to a building's home's heating, ventilation and air conditioning (HVAC) system, provide humidity to the building. Large humidifiers are used in commercial, institutional, or industrial contexts, often as part of a large HVAC system.
The need for humidifiers arises in low humidity environments which may occur in hot, dry desert climates, or indoors in artificially heated spaces. In winter, especially when cold outside air is heated indoors, the humidity may drop as low as 10-20%. This low humidity can cause adverse health effects for humans and animals within these environments either as workers, visitors, or residents. Industrial humidifiers may also be used when a specific humidity level must be maintained to achieve specific requirements such as preventing static electricity buildup or preserving material properties (e.g. art galleries, museums, libraries, and their associated storage).
Whilst evaporative humidifiers, natural humidifiers, vaporizing humidifiers, impeller humidifiers and ultrasonic humidifiers are all common types, it is the vaporizing humidifier (or vaporizer, steam humidifier, warm mist humidifier) that dominates the industry for most commercial humidification systems. These operate by heating or boiling water, releasing steam and thereby moisture into the air. However, today the vaporizing humidifier must achieve its desired function and operate with what are quite often conflicting requirements arising from different factors such as cost of ownership (CoO) and regulatory guidelines. From the CoO perspective owners seek high injection efficiency (so that the desired humidity within the air flow is achieved with minimal steam losses to condensate) together with low water consumption, and high energy efficiency to reduce energy consumption and running costs. Further, for low CoO the vaporizing humidifier should be high reliability, low maintenance, and simple to repair. Further, all of this is sought with variable humidification as the humidity level in an air flow unless it is dried first will vary throughout the day and through the year such that the humidity demand may range from nothing to all the desired level. Finally, the humidifier when exploiting a heat exchanger between a combusted material (e.g. natural gas, oil, etc.) should have low exhaust gas temperatures both from a safety/regulatory viewpoint but also from the desire to use reducing installation cost by using low temperature plastics for ducting/venting of the exhaust gases and high efficiency.
To date vaporizing humidifiers have been partially successful at achieving these requirements because they have utilized only a single stage heat exchanger that could not extract latent energy from exhaust gases because the secondary fluid is boiling water. The high temperature of the exhaust required use of high temperature stainless steel exhaust venting and resulted in potentially useful energy being simply exhausted into the atmosphere.
Accordingly, it would be beneficial to provide designs of dual-stage humidification systems with an effective design for achieving the conflicting objectives under variable humidification operation as well as addressing the control loop design of such dual-stage humidification systems.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.